Essentially Middle-Side (M-S) stereo is created using two microphones. A Mid microphone which is usually a cardioid mic is set to face the centre of the sound source and the Side microphone which is always a figure 8 pattern is set up at right angles to the Mid mic, as close as possible. This arrangement means that the null in the figure 8 pattern faces the sound source and the mic picks up information equally from the left and right side of the room. In itself if you play back these two mic tracks you get no sense of stereo at all and the secret is that to create a complete stereo image from Mid-Side signals, you have to ”decode" them with a special sum-and-difference matrix.

There are a number of Mid-Side decoders available as stand alone units or built into certain mic preamps but by far the easiest way to decode the signal is by using the a software decoder in a DAW like Cubase. When you have recorded the two tracks, simply duplicate the side Figure 8 mic track. Then pan these two tracks, one left and one right and put one of the pair out of phase with the phase button at the top of the track on the mixer page. Next link these two tracks together and as you add this pair to the centre mic which is panned dead centre and you get a very engaging and realistic stereo picture. Click on the video link at the top of the page to see a short video on how it's done.
Cubase also has a built in M-S decoder in the ToolsOne plug in but even if you aren't running Cubase you can get a great free M-S decoder, the Voxengo MSED. The Voxengo site has some great software plugins and the MSED is really good and it's free!

How much of the side pair you add is down to you and the beauty of M-S is that you can alter the stereo spread in post production. Adding more side information makes the stereo wider but you can also make the source seem closer by increasing the level of the mid microphone signal and you can even manipulate the stereo picture using the pan pots on the two side tracks. Obviously as you pan in the side channels the stereo image gets narrower and the mid-mic signal gets louder. Panning the left and right side-mic channels to dead centre (that is, to mono) effectively removes the side-mic signals and increases the mid-mic signal by 6 dB. Quite often when recording a singer I will use a M-S set to capture some of the room ambience and I always think that stereo miking captures a quality that a mono mic never quite achieves. Of course if it's not the effect you want in the mix then you can simply loose the side info. But give it a go on vocals and acoustic instruments. Even if you don’t have a decent figure 8 mic you can achieve the same effect using a pair of cardioids, one pointing left the other right as close to the centre mic as possible, I like Mid-Side stereo a lot and particularly with the mobile if the room is not great you can really play with the stereo image back in the studio. Just for a final note :try replacing the mid cardioid mic with an omni for a more spacious sound.